1. Field of the Invention
The present invention relates to an oxide powder for dielectrics, which can be used for a dielectric material, a method of manufacturing the oxide powder for the dielectrics, and a multi-layer ceramic capacitor comprising the dielectrics made using the oxide powder. More particularly, the present invention relates to an oxide powder for dielectrics having a perovskite structure made by a solid synthesis method, a method of manufacturing the oxide powder for the dielectrics, and a multi-layer ceramic capacitor comprising the dielectrics made using the oxide powder.
2. Description of the Related Art
An oxide of a perovskite structure, such as BaTiO3 or the like, has been generally used as a dielectric material for electronic components.
As an example of the electronic components using the oxide of the perovskite structure as the dielectric material, there is a multi-layer ceramic capacitor (MLCC).
When manufacturing the multi-layer ceramic capacitor, after sheets of dielectric layers are typically formed with internal electrodes therein by a printing method using internal electrode pastes, the sheets are laminated into multi-layers. Then, the internal electrodes and the dielectric layers in the multi-layers are sintered, and formed with external electrodes connected to the internal electrodes, thus imparting capacitance. Finally, in order to prevent problems from occurring in soldering, Ni and Sn layers are plated thereon, respectively.
With recent advances in high performance and miniaturization of electronic components, demands for miniaturization and increase in capacitance of the multi-layer ceramic capacitor have been increased.
As for a representative method for the miniaturization and increase in capacitance of the multi-layer ceramic capacitor, it has been attempted to decrease the thickness of the dielectric ceramic layer.
In order to decrease the thickness of dielectric ceramic layers, it is required to provide an oxide of a smaller particle diameter to an extent that at least 6˜7 particles can be contained in a dielectric ceramic layer. That is, in order to attain the miniaturization and the increase in capacitance of the multi-layer ceramic capacitor, it is needed to provide the oxide having the perovskite structure, such as BaTiO3 or the like, of the smaller particle diameter while having a high dielectric constant.
A method of manufacturing the oxide of the perovskite structure can be generally classified into the solid synthesis method and a wet method.
Generally, when the oxide of the perovskite structure, such as BaTiO3 powder, is made by the solid synthesis method, it is very difficult to control the particle size of the powder.
That is, there is a difficulty in producing the oxide powder of the perovskite structure having a smaller particle diameter with the solid synthesis method.
Meanwhile, in case of the wet method, although the oxide powder of the perovskite structure having the smaller particle diameter can be produced, there are problems in that manufacturing costs are high and in that the produced oxide powder has a remarkably low ferroelectricity and a low Curie Temperature.
Since the oxide powder produced by the wet method has a low C/A axial ratio (Tetragonality) of 1.005 or less, the dielectric constant is low.
Further, the oxide powder produced by the wet method contains 0.1˜3.0 wt % of OH− anion groups therein and defects such as pores, leading to reduction in density of the powder and finally reducing reliability of MLCC products.
An example of the method of manufacturing the oxide powder of a smaller particle diameter having the perovskite structure is set forth in Japanese Patent Laid-open Publication No. 2002-060219.
In Japanese Patent Laid-open Publication No. 2002-060219, BaTiO3 powder of the perovskite structure having a smaller particle diameter is synthesized by mixing a Ba-based hydroxide aqueous solution and a Ti-based hydroxide aqueous solution using the wet method, such as a hydrothermal synthesis method and a hydrolysis method.
According to the method disclosed in Japanese Patent Laid-open Publication No. 2002-060219, the BaTiO3 powder of a particle diameter of 0.2 μm level can be produced.
However, in the BaTiO3 powder, there are problems in that the oxide contains the OH− anion groups of about 0.1˜3.0 wt % and the defects, such as pores, in the particles.
When the defects are present in the powder, the powder has a low density and a decreased crystallinity. Further, the C/A axial ratio is also low, so that the oxide has the perovskite structure, such as cubic BaTiO3.
Thus, when manufacturing the oxide powder of the perovskite structure with a smaller particle diameter using the method of Japanese Patent Laid-open Publication No. 2002-060219, there are problems in that a sufficient ferroelectricity is not exhibited, the reliability is decreased, and the manufacturing costs are increased.
As an example of the solid synthesis method, a method of manufacturing the oxide powder of the perovskite structure, for example, BaTiO3, is set forth in Japanese Patent Laid-open Publication No. 2002-234769.
In the method disclosed in Japanese Patent Laid-open Publication No. 2002-234769, the BaTiO3 powder with a smaller particle size is synthesized through an improved mixing process, which uses titanium oxide (TiO2) and barium carbonate (BaCO3) of a mono-disperse particle size and a large specific surface area, respectively.
The method synthesizes the BaTiO3 powder of the perovskite structure using the solid synthesis method, so that factors of the defects, such as pores, in the particle are thoroughly removed and so that the manufacturing costs are reduced.
However, in case of the method disclosed in Japanese Patent Laid-open Publication No. 2002-234769, although the powder having a particle size of 0.2 μm or less can be produced, intermediate phases, such as Ba2TiO4 or BaTi3O7, additionally remain in the powder.
Thus, in order to remove the intermediate phases, the powder should be calcined at a high temperature of 1,200° C. or more, or calcined by controlling a molar ratio of the materials.
However, when the powder is calcined at a high temperature of 1,200° C. or more as described above, even though a high crystallinity of the powder can be attained, grain growth occurs, making it difficult to provide the mono-disperse BaTiO3 having a particle size distribution of D99/D50<4.0 and having an average particle diameter of 0.2 μm or less.
The term “D50” means a diameter of a particle in the 50th percentile of the volumes of particles within powders, and the term “D99” means a diameter of a particle in the 99th percentile of the volumes of particles within powders.